The trend in semiconductor technology to double the functional complexity of its products every 18 months (Moore's “law”), which is still valid today after having dominated the industry for the last three decades, has several implicit consequences. First, the cost per functional unit should drop with each generation of complexity so that the cost of the product with its doubled functionality would increase only slightly. Second, the higher product complexity should largely be achieved by shrinking the feature sizes of the chip components while holding the package dimensions constant; preferably, even the packages should shrink. Third, the increased functional complexity should be paralleled by an equivalent increase in reliability of the product. And fourth, but not least, the best financial profit rewards are held out for the ones who are ahead in the marketplace in reaching the complexity goal together with offering the most flexible products for application.
The scaling of the components in the lateral dimension requires vertical scaling as well, so as to achieve adequate device performance. This vertical scaling requires the thickness of the gate dielectric, commonly silicon dioxide (SiO2) to be reduced. Thinning of the gate dielectric results in increasing domination of the electrical characteristics by interface effects, and increased leakage of the dielectric due to nontrivial quantum tunneling effects. Therefore, the industry has moved to gate dielectric materials with a larger dielectric constant, or “k,” allowing the use of a thicker dielectric while maintaining the same degree of capacitive coupling to the transistor channel.
One material suitable for high-k gate dielectrics is hafnium silicate, ideally represented by its empirical formula HfSiO4. HfSiO4 offers a high degree of compatibility with current semiconductor manufacturing processing, and may be formed using a chemical vapor deposition (CVD) process in conventional manufacturing tools. However, a typical metal-containing film produced by a CVD process has nonuniformities of the constituent elements that result from inherencies in the CVD process as well as interfacial effects. Thus, for example, a hafnium silicate film may have a greater concentration of hafnium, herein referred to as [Hf], at the top surface than at the interface with the substrate. Moreover, the decrease of [Hf] with depth may not be monotonic.
A gradient in the concentration of elemental constituents in a gate dielectric may have an undesirable impact on the performance and reliability of a device using the gate dielectric. For example, a transistor using a material layer with such nonuniformities as a gate dielectric may experience reduced breakdown voltage and operating lifetime, as well as threshold voltage shifts as a result of charge trapping and leakage in the gate dielectric.
In certain semiconductor devices, a manufacturing process synonymously referred to as a “split-gate” or “dual-voltage” process is used to produce transistors with different gate thicknesses, and therefore different gate threshold voltages. Such a process is useful, for example, in devices that require a low-voltage transistor in a high performance “core,” and a higher voltage transistor for I/O transistors used to interface the core to external circuitry. In such a device, a core transistor may have a gate thickness of about 1 nm, while I/O transistors may have gate thickness of about 3 nm. In these devices, the performance and reliability issues related to nonuniformities in the gate dielectric are generally not significant in the I/O transistors because the properties of the gate dielectric are dominated by bulk properties. However, these issues cannot be neglected for the core transistors, as the properties of the thinner gate are significantly impacted by the surface effects. Thus, it can be expected that as semiconductor device geometries continue to shrink, the aforementioned performance and reliability issues will become increasingly significant as the industry moves to metal-containing gate dielectric materials for high-performance transistors.
An urgent need exists, therefore, for a method of improving the uniformity of elemental constituents used in comparatively thin gate dielectrics.